Fix audio bug causing infinite loops in SR

Fix bug where SACT2.Music_IsPlay would return true after a stream
stopped playing. This was causing infinite loops in Sengoku Rance when
waiting for a fade-out to complete.

src/audio_mixer.c

@@ -242,6 +242,10 @@ static int refill_stream(sts_mixer_sample_t *sample, void *data)
 		}
 	}
 
+	if (r == STS_STREAM_COMPLETE) {
+		ch->voice = -1;
+	}
+
 	return r;
 }